121
Synthesis and Polymerization of Bridgehead-Substituted Bicyclobutanes H. K. Hall, Jr., C. D. Smith, E. P. Blanchard, Jr.,l S . C. Cherkofsky,* and J. B. Sieja Contribution No. 1624 f r o m the Central Research Department, Experimental Station, E . I . du Pont de Nemours and Company, Wilmington, Delaware 19898. Received March 12, 1970
Abstract: Bicyclobutanes carrying ester, acid, amide, and acyl functions at the bridgehead were synthesized. When the other bridgehead was unsubstituted or, in one case, carried an ester group, free-radical polymerization was successful, However, a 3-methyl or 3-phenyl group suppressed polymerization. It was advisable to use a mild chain-transfer agent to avoid gel. In contrast, bicyclobutanes carrying only hydrogen, phenyl, or methyl at the bridgehead were not successfully polymerized. They did not copolymerize with vinyl monomers, giving only gelled and highly branched products. We concluded that if the bridgehead substituent can stabilize an adjacent free radical, propagation occurs; otherwise, the hot radical abstracts tertiary hydrogen from adjacent chains. The hydrocarbons did copolymerize with SO?to give polysulfones. A few reactions and calculations of isomerization equilibria for these compounds are reported.
T
he preceding article? showed that l-bicyclobutanecarbonitriles, polymerizing through the strained 1,3 bond, constituted a new class of reactive monomers. To explore the effects of bridgehead substituents on polymerization, we have studied the synthesis and polymerization of a variety of substituted bicyclobutanes. Esters. 3-Chlorocyclobutanecarbonitrile2 readily underwent hydrolysis by hydrochloric acid to form 3chlorocyclobutanecarboxylic acid. Direct esterification of the chloro acid and dehydrochlorination formed methyl 1-bicyclobutanecarboxylate. 3 As in the case of the chloronitriles, the 3-chloro group was amply reactive in the cyclization step. Other chloro esters
ro-3-methylcyclobutanecarboxylic acid. Esterification with methanol or isobutylene gave the corresponding esters. Dehydrochlorination gave the corresponding bicyclobutanes. C02CH3
COOC,H,-tert
COOCH,
COOC,H,-tert
I
I
I
COOCH3 I Cl+C02H
C l U C C C l
C1u C O O C H 3
\
C1~COOCIHg-tert COOC,H,-tert I
Reaction of 3-oxocyclobutanecarboxylic acid5 with excess phenylmagnesium bromide gave 3-phenyl-3hydroxycyclobutanecarboxylic acid, which on successive treatment with concentrated hydrochloric acid, diazomethane, and sodium hydride gave methyl 3-phenyl-1-bicyclobutanecarboxylate. O=@COOH
-
CsH5 c1e C O O H
were formed when the acid chloride was treated with tert-butyl alcohol or neopentyl alcohol. Dehydrochlorination gave the corresponding bicyclobutane esters. 3-Methylenecyclobutanecarbonitrile was converted by refluxing concentrated hydrochloric acid to 3-chlo(1) Chestnut Run Film Department Laboratory, E. I. du Pont de Nemours and Co., Wilmington, Del. 19898. (2) See preceding article: H. K. Hall, Jr., E. Blanchard, Jr., S. Cherkofsky, J. Sieja, and W. Sheppard, J . Amer. Chem. Soc., 93, 110 (1971). (3) (a) K. B. Wiberg, Rec. Chem. Progr., 26, 143 (1965); (b) K. B. Wiberg, Tetrahedron, 21, 2749 (1965).
-
CGH5 X>COOH HO CH C1‘ e C O O C H ,
-
c6H5
1-Chloro-1,3-cyclobutanedicarbonitrile2 was converted with methanol and anhydrous hydrogen chloride t o dimethyl l-chlorocyclobutane-1,3-dicarboxylate (4) We are indebted to Dr. A. Cairncross for this synthesis. The methyl 3-methylbicyclobutanecarboxylatewas prepared independently by Wiberg and coworkers.3 (5) J. D. Roberts and F. F. Caserio, J . Amer. Chem. SOC.,79, 5837 ( 1957).
Sieja, et a/.
Polymerization of Bridgehead-Substituted Bicyclobutanes
122
Treatment with base gave dimethyl 1,3-bicyclobutanecarboxylate.6
1,1,3,3-tetrachlorocyclobutanelo as follows. Recently,
COOCH,
CH,OOC
WCOOCH,
c1
COOCHj
Acids and Amides. Saponification of methyl 1bicyclobutanecarboxylate led to the corresponding sodium salt and thence t o the crystalline free acid. Base-catalyzed peroxide converted l-bicyclobutanecarbonitrile into the corresponding carboxamide. 3Methylbicyclobutane-1-carboxamidehad been prepared earlier by an analogous reaction.'
Ketones. Reaction of 3-hydroxycyclobutanecarbonitrile with excess methylmagnesium iodide gave 3acetylcyclobutanol. This was converted to the sulfonate, the iodide, and t o 1-acetylbicyclobutane.
-
HO+CN
C,H,SO,O+COCH~
-
HO+COCH~
-
I-+-COCH~
-
COCHJ
I
Reaction of 3-methylenecyclobutanecarbonitrilewith phenylmagnesium bromide, followed by hydrogen iodide and NaH, gave 3-methyl-1 -benzoylbicyclobutane.
-
CH,+CN
CH&--COC,H.,
% COC,H,
five 2,2,4,4-tetramethylbicyclobutaneshave been prepared but not isolated.8s11-15 Homopolymerizations. The bicyclobutanes were subjected to radical, cationic, anionic, and coordination polymerization conditions. Only the radical polymerizations gave good yields of high polymers. These experiments are summarized in Table I. The bicyclobutanes with electronegative substituents (COOR, CONH2, COCH3) at the bridgehead polymerized well under free-radical conditions. Methyl 1-bicyclobutanecarboxylate3 and methyl 2,2,4,4-tetramethylbicyclobutanecarboxylate' polymerize when left at room temperature. It was necessary in some cases t o add a weak chain-transfer agent like isobutyraldehyde to avoid some gel formation. 1-Vinylbicyclobutane also polymerized but, unexpectedly, the 1-phenyl derivative did not. 3-Methyl or 3-phenyl substituents on methyl 1-bicyclobutanecarboxylate suppressed polymerization but a 3-methoxycarbonyl substituent could be tolerated. Bicyclobutanes with only methyl or hydrogen at the bridgehead did not polymerize well. 1-Methylbicyclobutane gave oligomers with BF3. Irradiation with uv light converted bicyclobutane and 2,2,4,4-tetramethylbicyclobutane to cross-linked polyhydrocarbons in low conversions. The former gave oligomers with r-allyl nickel bromide, and dimers with other coordination or cationic initiators. The dimer mixture contained olefinic and cyclopropane hydrogens by nmr and contained at least four dimers by gc and mass spectral analysis. Copolymerizations with Vinyl Monomers. Bicyclobutanes with groups which are particularly good at stabilizing radicals copolymerized smoothly with vinyl monomers t o yield copolymers containing 1,3-cyclobutane rings in the chain. Similar copolymers with Y = CN had been prepared previously.2 The 1-phenyl -CH,CH+
CH3
Hydrocarbons. Bicyclobutane3 and its 1-phenyl and 1-vinyl derivatives were available uia the cycloaddition of ketene t o vinyl ethers investigated by Sieja.8 Decarboxylative chlorinationg of 3-methyl-3chlorocyclobutanecarboxylic acid gave the 1,3-dichloride, which with sodium furnished l-methylbicyclobutane. 2,2,4,4,-Tetramethylbicyclobutane was preCOlH
p,
pared from the readily accessible 2,2,4,4-tetramethyl(6) This compound was first prepared another way by Dr. C. E. Coffey of the du Pont Explosives Department, who we thank for instructions and samples. (7) E. P. Blanchard, Jr., and A. Cairncross, J . Amer. Chem. Soc., 88, 487 (1966). (8) See'accomganying article: J. B. Sieja, ibid., 93, 130 (1971). (9) J. Kochi, ibid., 87, 2500 (1965).
Journal of the American Chemical Society
C1
Cll
-+
xI
ir
derivative did not copolymerize with styrene under radical or anionic conditions. Bicyclobutane copolymerized with acrylonitrile or methyl methacrylate to give cross-linked copolymers. Bicyclobutane and its 1-methyl derivative copolymerized readily with sulfur dioxide to give 1 : 1 polysulfones. Table I1 summarizes the results. Copolymerizations of Two Bicyclobutanes. Freeradical copolymerizations of bicyclobutanes carrying electronegative substituents with one another were accomplished readily to yield copolymers containing 1,3-~yclobutane links only. The cyano compounds (10) H. G. Gilch, J . Org. Chem., 30, 4392 (1965). (11) M. R. Rifi, J . Amer. Chem. Soc., 89,4442 (1967). (12) D. P. G. Hamon, ibid., 90,4513 (1968). (13) C. Burridge and D. P. G. Hamon, Chem. Commun., 206 (1968). (14) M. F. Neumann, Angew. Chem., I n t . Ed. Engl., 6, 79 (1967). (15) E. J. Corey and M. Jautelat, J . Amer. Chem. Soc., 89, 3912 (1967).
1 93:I / January 13, 1971
123 Table I. Homopolymerizations of Bicyclobutanes Monomer Methyl bicyclobutane1-carboxylate Methyl bicyclobutane1-carboxylate Methyl bicyclobutane1-carboxylate Methyl biciciobutane1-carboxylate Methyl bicyclobutane1-carboxylate
Amt, g
Solvent
Amt, ml
Polymerized spontaneously 1.O Hz0 10 0 . 6 HtO Soap soln 3 . 2 DMSO
3.5 0.7 10
3 . 2 DMSO
10
Methyl bicyclobutane- 1 . 0 Hexamethylphosphor1-carboxylate amide 4.0 2-Butanone tert-Butyl bicyclobutane-1-carboxylate 1 . 3 Tetramethylene Methyl 3-methylsulfoxide bicyclobutane-lcarboxylate 1 . 3 Tetramethylene tert-Butyl 3-methylsulfoxide bicyclobutane-1carboxylate Dimethyl 1,3-bicyclo- 4.0 H20 butanedicarboxy late
5
10 14 14 50
Dimethyl 1,3-bicyclo- 3 .O butanedicarboxylate Bicyclobutane-10.72 DMSO carboxylic acid
4
Bicyclobutane-lcarboxamide
1.08 DMSO
6
Bicyclobutane-1carboxamide
1 . 0 DMSO
2
1-Acetylbicyclobutane 7.8 H10
150
1-Acetylbicyclobutane 5 .O DMSO a
0.1
25
Initiators
Amt, Temp, Time, Yield, 8 "c hr g
?inha
Comments Sparkling clear tough plug
Standard emulsion recipe Azobisisobutyronitrile Azobisisobutyro- 0 . 1 nitrile Azobisisobutyro- 0 . 1 nitrile Isobutyraldehyde 0.25 (ml) NaH soln 0.2 Azobisisobutyro- 70 nitrile (mg) a,a'-Azobis((~,~-0.05 dimethylvalcronitrile) a,a'-Azobis(a,y- 0.05 dimethylvaleronitrile) Dipotassium 0.22 phosphate 3 % aq polymeth- 0.43 acrylic acid 0.18 M Vazo in 0 . 3 methanol (ml) Di-tert-butyl per- 37 b" oxide Azobisisobutyro- 0.05 nitrile Lsobutyraldehyde 0.1 (ml) Azobisisobutyro- 0.05 nitrile
Potassium tertbut oxide
50
3.5
1.0
4.86
5.60
50
16
0.6
50
16
High
50
16
2.5
Rigid gel 1.60
Room 16 temp
High 0.63 Brittle hazy film from CHCls
24 4
High 0.23 DTA, endotherm 1312140°, dec 220-240" 0.95 0.70
4
0.90 0.74
atm u p to 160
3
2.29 2.55
65
1
High
50
16
60-65 75
6Ooo atm 75
6Ooo
Tough sparkling colorless solid plug 0.45 4.29 Inherent viscosity 0.1 in 0.5% NaOH at 25
x
O
60
16
0.05 60
2
Standard emulsion 50 1 recipe Benzoyl peroxide 0.01 Room 24 temp
x
1.05 6.63 Inherent viscosity 0.1 in formic acid at 25" Ir only primary amide 0.1 Ir secon~ C O N H dary amide 3.5 1.12
dn
3.0
2.34
in chloroform unless otherwise noted.
prefers t o abstract the reactive tertiary hydrogens of the polymer. Dimerization of such radicals leads t o cross-linking and gel formation. prepared earlier were particularly facile. Copolymerizations with bicyclobutane led t o highly cross-linked poi ymers. We think that successful free-radical polymerization depends on the competition between propagation and chain transfer to polymer. If the substituent is cyano, the growing radical is relatively stable and discrimCN
+CN
+ @
--t
propagation
inating, and propagation t o a new stable radical is favored. Also, the reactivity of the tertiary hydrogens in the polymer chain is lowered by the adjacent cyano group. As the substituent is less able to stabilize the radical (in the extreme, bicyclobutane itself), the unstabilized radical is not discriminating, has little tendency to propagate to a new "hot" radical, and
Isomerizations. During attempts t o polymerize 1phenylbicyclobutane we observed ready isomerization t o I-phenylcyclobutene. This raised the question of the exact position of equilibrium in such reactions. Heats of formation for bicyclobutane and cyclobutene1+'* show that the latter is much more stable. (16) K . B. Wiberg and R. A. Fenoglio, J . Amer. Chem. SOC.,90, 3395 (1968). (17) A. D a d , J . Chem. Phjms., 27, 1227 (1957). (18) Nut. Bur. Stand. (U.S.) Circ., No. 500, 477, 527, 571 (1952).
Sieja, et 01.
Polymerization of Bridgehead-Substitured Bicyclohutanes
124
c
c
